The present invention is a process for producing a distillate fuel heavier than gasoline. In particular, it is a process to optimize the production of a distillate from a hydrocarbon synthesis process. The use of Fischer-Tropsch (hydrocarbon synthesis) liquids as pure or as a component of distillate fuels is well known in the art. The products of the Fischer-Tropsch synthesis are predominantly normal paraffins. Economically it is desirable to operate Fischer-Tropsch catalysis at the highest possible Schulz-Flory alpha, in order to minimize undesirable light paraffins. High alpha operation over high activity cobalt catalysts, results in a high boiling, paraffinic wax product that is unsuitable for direct distillate blending. A high quality diesel blend stock is typically produced from the high Schulz-Flory product using hydroisomerization and or mild hydrocracking of the 700.degree. F..sup.+ wax. Sie, S. T. [Catalysis Letters 1990, 7, 253-270], invokes the hydroconversion of the entire hydrocarbon synthesis product. This hydroconversion results in 100% paraffinic products, although the degree of branching may vary. One of the great advantages of Fischer-Tropsch derived diesel fuels is their high inherent cetane number. There is a great incentive to maximize the cetane of the fuel in order to increase its value as a diesel blend stock, however, the product diesel must also meet any appropriate cold flow specifications, such as diesel cloud point or cold filter plugging point (CFPP). High cetane number corresponds with high molecular weight and low levels of branching, while cold flow often requires lower molecular weights and high levels of branching. Optimization of these two properties, either in blending or in actual plant operation is unwieldy due to the time consuming nature of both engine cetane and CFPP determinations. The present invention uses .sup.13 C NMR to rapidly determine both cetane and cold flow properties. These determinations are then used to optimize both product blending and unit operation. More detailed information about the molecular structure is also provided by the .sup.13 C NMR analysis and can serve as a valuable process diagnostic.